The invention relates to a method for cold-starting a fuel cell system of a motor vehicle, in particular at temperatures below freezing point, comprising at least one heating-up phase and a warm-up phase. The invention further relates to a fuel cell system of a motor vehicle.
Starting a cold fuel cell system below operating temperature, in particular at low temperatures below freezing point, poses a well-known problem in a fuel cell vehicle.
After operating the ignition, a fuel cell vehicle can only drive off if a control unit issues a drive enable signal. This drive enable signal can, however, be issued only after the fuel cell system has been heated to an adequate operating temperature.
In order to make a cold start as energy-efficient as possible, PCT International Publication No. WO 20081445 A2 discloses a fuel cell system with a cold start detection device and with a load that can be connected to the fuel cell stack. The control unit is designed such that the connected load can be varied with one or more load steps in response to the detection of the cold start state of the fuel cell stack. The load step may follow the drive enable signal in the dynamic operation of the vehicle, or it may precede the drive enable signal during a heating-up phase. This heating-up phase may, for example, have a duration of 120 seconds.
Unfortunately, the number of loads that can be connected to the fuel cell stack is limited, in particular in mobile applications such as a motor vehicle, and owing to operational limits the electric loads are not available without restriction as power sinks.
Exemplary embodiments of the present invention are direct to increasing the operational availability of the motor vehicle.
According to the invention, the operational availability is increased by consuming a defined quantity of an output power of the fuel cell system in the heating-up phase and/or in the warm-up phase using a power consumption means and by converting the consumed quantity of the output power into power loss by impressing a suitable current into at least one winding of an electric motor that is to be energized.
The output power of the fuel cell system is defined as the electric power of the fuel cell stack minus the power consumption of the electric secondary loads that are required for the operation of the fuel cell system and/or of the fuel cell vehicle. Electric secondary loads may for example be one or more auxiliary drive(s) of the fuel cell system, an HV auxiliary heater, possibly at least one low-voltage DC/DC converter, as well as a low-voltage electrical system and low-voltage loads such as cab air heaters or vehicle lighting.
Such a method therefore provides for a higher loading of the fuel cell system, which is desirable during a heating-up and/or warm-up phase, by selecting a component (power consumption means) already present in a fuel cell vehicle and by converting the impressed current into a further, likewise already existing, component (at least one phase winding of an electric motor). The execution of the method according to the invention advantageously results in a considerable reduction of the duration of the current operating phase in which the method is used, so that an operating phase following this operating phase can be initialized earlier. If the method according to the invention is used during the heating-up phase, the start of the following warm-up phase can be advanced considerably. As the drive enable signal is, on activation of the warm-up phase, issued by an algorithm of a control unit, the driver is able to drive off already, subject to some restrictions.
If, for example, the power demand of a traction motor falls below a defined value during a low-load or idling operation, this value being an indicator that a higher output power of the fuel cell system is available than the drive power which can be consumed by a traction motor, it is useful if the method according to the invention is used during the warm-up phase, in order to load the fuel cell stack as highly as possible. As a result, the warm-up phase can be terminated earlier, and the vehicle is fully operational at an earlier time.
The application of the method according to the invention, however, involves a reduction of energy efficiency, because the consumed quantity of the output power is converted into power loss. The consumed quantity of the output power that is converted into power loss therefore makes virtually no contribution to the effective power of the electric motor, but it significantly reduces the duration of the cold start or of the operating phase in which this method is used.
An inverter (DC/AC converter) can be used as a power consumption means because the inverter has several half-bridges that can be selected in such a way that the consumed quantity of the output power of the fuel cell system can be converted into power loss at one or more phase winding(s) of the electric motor.
In one embodiment of the invention, the drive enable signal is issued in the warm-up phase in dependence on the output power of the fuel cell system. In this way, an operating point for issuing the drive enable signal can be found advantageously as an agreement of the vehicle objectives, such as ruggedness, acceleration and service life of the fuel cell system. The drive enable signal is advantageously issued at an availability of less than 50% of the output power of the fuel cell system, in particular at an availability of 20% of the output power. Depending on the dimensioning of the vehicle, the warm-up phase can begin within less than 15 seconds after the heating-up phase when using the particularly preferred embodiment, in which the drive enable signal is issued at 20% of output power.
In an alternative embodiment of the method according to the invention, a drive enable signal is issued in the warm-up phase in dependence on the available total output power of a fuel cell stack and an energy storage device. The total output power results from the addition of the available power of the fuel cell stack and the available power of the energy storage device minus the power consumption of the electric secondary loads that are required for the operation of the fuel cell system and/or of the fuel cell stack. In this way, not only the capacity of the fuel cell stack, but also the capacity of the energy storage device is taken into account in a cold start in an advantageous way. Depending on the choice of energy storage device, this can be very limited at low temperatures. The drive enable signal is advantageously issued in the warm-up phase at an availability of less than 50% of total output power, in particular at an availability of 20% of total output power.
In a further development of the invention, the temperature in the cooling circuit of the fuel cell stack is increased and/or the input humidity of the supply gas for the fuel cell stack is reduced, preferably on the cathode side, in order to dry the fuel cell stack, in particular on completion of the cold start, for a subsequent start.
In a variant of the method according to the invention, the fuel cell stack and/or the energy storage device is/are during the heating-up phase and/or during the warm-up phase preferably periodically loaded with at least one electric load that is variable in terms of its power consumption, so that the fuel cell stack and/or the energy storage device output(s) a higher power in a first time interval than in a second time interval. Depending on the type of fuel cell and/or energy storage device, this may result in an earlier operational availability of the fuel cell stack and/or of the energy storage device.
Owing to an operating temperature that is lower than that of other fuel cell types, polymer electrolyte membrane fuel cells are preferably used as fuel cell type. The energy storage device is, for example, a lithium ion battery or a nickel metalhydride battery. Compared to other electrochemical energy storage devices, the lithium ion battery offers the advantage of a relatively high energy content. According to prior art, the nickel metalhydride battery offers the advantage of lower costs compared to the lithium ion battery.
In a particularly preferred embodiment, the power output of the fuel cell stack and/or of the battery is adjusted such that during the heating-up phase and/or the warm-up phase the fuel cell stack and/or the energy storage device output(s) a power of 0 kW in the second time interval. The temperature fluctuations resulting from the alternating currentless and current-output operation can advance the operational availability of the fuel cell stack and/or the energy storage device even more.
In an alternative variant, both the fuel cell stack and the energy storage device are periodically loaded with at least one electric load that is variable in terms of its power consumption, so that the fuel cell stack outputs a higher power in a first time interval than in a second time interval and the energy storage device outputs a lower power in the first time interval than in the second time interval. The fuel cell stack advantageously outputs a power of 0 kW in the second time interval, while the energy storage device outputs a power of 0 kW in the first time interval. By alternately shifting the power ratios, a faster heating-up process can be achieved both for the fuel cell stack and for the energy storage device, which in turn increases the ruggedness of the system as a whole and the availability of the vehicle.
The fuel cell system according to the invention can be connected in an electrically conductive manner to an intermediate circuit, which in turn can be connected in an electrically conductive manner to the electric motor via the inverter.
The electric motor is preferably designed as a traction motor. Depending on its power rating, the traction motor can convert a larger proportion of the output power of the fuel cell stack into power loss than an auxiliary drive. As an alternative, the electric motor is designed as an auxiliary drive, such as an air compressor, an anode recirculation fan or a water pump.
In a further development of the invention, the intermediate circuit can be connected in an electrically conductive manner to at least one further electric motor via at least one further inverter. For each further electric motor, one further inverter is preferably provided, and a further inverter is assigned to each further electric motor. In addition, the intermediate circuit can be connectable in an electrically conductive manner to further electric machines. A control unit is provided for selecting the at least one further inverter for the purpose of consuming a further quantity of the output power of the fuel cell system and for converting the consumed further quantity of the output power into power loss by impressing a suitable current into at least one winding of the at least one further electric motor which is to be energized. In this variant, the electric motor is preferably designed as a traction motor, and the at least one further electric motor is preferably designed as an auxiliary drive of the fuel cell system. As explained above, the traction motor is usually capable of converting a larger proportion of the output power of the fuel cell stack into power loss than an auxiliary drive. In a fuel cell vehicle, however, the number of auxiliary drives usually exceeds the number of built-in traction motors. It is therefore also advantageous if output power can be converted into power loss in the auxiliary drives as well.
In a further development of the invention, the energy storage device can be connected in an electrically conductive manner to the intermediate circuit via a first DC/DC converter, so that the voltage of the intermediate circuit can be controlled largely independently of the voltage level of the energy storage device.
The first DC/DC converter can preferably be operated bidirectionally, enabling current to flow both from the energy storage device into the intermediate circuit and from the intermediate circuit into the energy storage device. This may, for example, involve recuperation processes for recycling kinetic braking energy or a charging of the energy storage device by the fuel cell stack.
The first (bidirectionally operated) DC/DC converter is particularly preferably designed as a voltage reduction/step-up converter combination. The first DC/DC converter is advantageously capable of adjusting an intermediate circuit voltage to a very low value, in particular below 200 V, during a heating-up phase and/or a warm-up phase. As the fuel cell stack is directly coupled to the intermediate circuit in an electrically conductive manner, the setting of a low intermediate circuit voltage results in a low fuel cell stack voltage and therefore in poor efficiency and high direct heat conversion in the fuel cell stack. The higher the heat input, the lower the intermediate circuit voltage.
In an alternative variant, the DC/DC converter is located between the battery and the intermediate circuit. In addition, a second DC/DC converter, which is designed as a step-up converter or as a combination of voltage reduction and step-up converter, is provided between the intermediate circuit and the fuel cell stack. The second DC/DC converter is provided for adjusting the fuel cell stack voltage to as low a value as possible, preferably below 75 volts, during a heating-up phase and/or during a warm-up phase. With the alternative variant (with the second DC/DC converter), a much lower fuel cell stack voltage can be set than with the particularly preferred variant with only one (first) DC/DC converter, which is desirable during the heating-up phase and/or during the warm-up phase. The particularly preferred variant, however, offers the advantage of lower costs, because the second DC/DC converter is omitted.